1. Field of the Invention
The present invention relates to an arithmetic unit used in e.g., a digital still camera, a facsimile telegraph, a color copying machine, a visual telephone, etc. and performing an orthogonal transformation such as a discrete cosine transformation (called a DCT or a DCT-transformation in the following description) and a discrete sine transformation (called a DST or a DST-transformation in the following description) for compressing and extending a color image.
2. Description of the Related Art
Various kinds of systems for coding a color still image are generally searched and developed. The adoption of an adaptive discrete cosine transformation (called an ADCT or simply a DCT in the following description) was determined as an international standard of the above coding systems by Joint Photographic Export Group (called JPEG in the following description) in October, 1989. A processing of this adaptive discrete cosine transformation will next be schematically described.
In DCT-processing and IDCT-processing, it is necessary to perform 64 multiplying operations with respect to all input data and DCT-processing coefficients composed of an (8.times.8) matrix, and perform 56 adding operations with respect to the multiplied results. Accordingly, a processing time required to perform ADCT-processing including a memory operation with respect to a memory is longer than a processing time from a charge coupled device to one of page buffers. Therefore, it is necessary to dispose a memory for temporarily storing color and brightness information of one photographed image picture, circuit elements corresponding to the respective page buffers, and a frame memory. For example, in a still video camera, it is necessary to dispose a memory having a capacity of 7.5M bits with respect to one picture. Accordingly, there is a problem that the size of a circuit structure of the entire orthogonal transformation arithmetic unit is increased.
When an image is generally coded and coded data are reproduced as an original image, there is a case in which the size of an image obtained by the coded data is reduced or enlarged in comparison with the original image. In such reducing and enlarging processings, inverse orthogonal transformation processing is performed with respect to orthogonally transformed data and these data are reproduced as the original image. Thereafter, the reducing and enlarging processings are performed with respect to the reproduced data. When the image is reproduced and the reducing and enlarging processings are further performed with respect to the reproduced image, it is necessary to separately dispose a processing unit for performing the reducing and enlarging processings. Further, there is a problem that a processing time is further increased by separately disposing such a processing unit for performing the reducing and enlarging processings.
Further, the general orthogonal transformation arithmetic unit for performing DCT-processing and inverse DCT-processing has a DCT-processing section and an inverse DCT-processing section. Two main arithmetic circuits similarly operated, etc. are disposed in the DCT-processing section and the inverse DCT-processing section. An area for these main arithmetic circuits amounts to about 90 percent of an area for a semiconductor circuit chip constituting a DCT/inverse DCT processor. Therefore, the size of a circuit structure is increased by these two main arithmetic circuits.
Accordingly, the circuit structure is large-sized in the general orthogonal transformation arithmetic unit for performing the DCT/inverse DCT-processings.
To change data compressibility, it is sufficient to change the number of nonzero values with respect to quantized image component data. Namely, it is sufficient to move a zero boundary region having only quantized value zero and change a quantizing coefficient .alpha..
When the quantizing coefficient .alpha. is set to a value close to zero, the quality of a reproduced image is improved, but the amount of image data is correspondingly increased, thereby reducing the data compressibility. The reduction of the data compressibility means that the data compressibility is bad and close to one. In contrast to this, when the quantizing coefficient .alpha. is close to one, the quality of a reproduced image is reduced, but the data compressibility is improved. Thus, it is possible to determine superiority or inferiority of the quality of the reproduced image by changing the quantizing coefficient .alpha.. When the data compressibility is reduced, the number of sheets of still images stored to one memory medium is reduced although the image quality is improved. In contrast to this, when the data compressibility is increased, the number of sheets of images stored to the memory medium can be increased although the image quality is reduced.
The above description relates to one image block within one image. When the above-mentioned processing is performed with respect to the one image, the above-mentioned operations are performed with respect to 5400 image blocks constituting the one image.
The above description relates to a process for storing a photographed image to the memory medium. A reproduced image with respect to such a stored image can be obtained by a process completely reverse to the image storing process.
In a general apparatus for compressing and extending a color image by using the DCT-processing, there is no orthogonal transformation arithmetic unit having the relation between the quantizing coefficient .alpha., the image quality and the number of sheets of stored images.